(CNN) -- Scientists have learned that the moon is nearly 4 feet further from us now than it was when Neil Armstrong first set foot on it, thanks to a venerable experiment he and Buzz Aldrin placed on the lunar surface 30 years ago.

From the perspective of scientists, that finding is one of more mundane returns from the Lunar Laser Ranging Experiment, says astronomer Peter Shelus of the University of Texas, Austin.

The McDonald Laser Ranging system in Texas shoots laser beams from an optical telescope to reflectors on the lunar surface.

"One of the most exciting things coming out of this right now is that relativists around the world agree that lunar ranging is one of the few techniques which can really give us a good handle on several gravitational things -- for instance, is the force of gravity changing with time?" Shelus says.

So far, the answer to that question is no -- the force of gravity has remained constant in the past 30 years, but the jury is still out for the future, Shelus says.

Astrophysicists have used the lunar ranging experiment to test some of their most complex theories, including Albert Einstein's Theory of General Relativity.

The premise of the experiment is simple.

It relies on four reflectors, the first of which was placed on the surface of the moon by Apollo 11 astronauts Armstrong and Buzz Aldrin. The reflectors, about the size of flat computer screens, almost could be made by a child. Each is a half-meter-diameter mosaic of fused silica cubes sliced at the same angle and affixed to a tilted aluminum sheet.

The cubes act as prisms and have the special property of reflecting light directly back to its point of origin.

Apollo 14 and 15 astronauts placed similar reflectors elsewhere on the moon, as did a Russian lander.

For the past 30 years, astronomers at observatories in Texas and France have worked in tandem like a global surveying team, shooting laser beams from optical telescopes at the reflectors to measure how quickly the light bounces back. Night by night, they have added to what has become a massive, longitudinal data set on the Earth-moon system.

The laser beams bounce back anywhere from 2.3 to 2.6 seconds later and it is the tiny time differences that reveal the secrets of the moon's orbit and its movement over time.

By multiplying the time the laser takes to return by the speed of light, researchers can calculate how far away the moon is at a particular point in time.

Jim Williams, a research scientist at NASA's Jet Propulsion Laboratory, is one of about 50 researchers worldwide who works with the data.

He made a splash in the lunar science community in recent months with an analysis of lunar ranging data that showed the moon has a liquid core. Most scientists agree that the moon has a core, but Williams says he is the only one to claim it is liquid.

"It's hard to know what's going on inside a body," he says. "And it turns out that the seismic waves (used to study the moon during later Apollo missions) couldn't get into that central area. It got deep but not deep enough, so there's been a kind of mystery about what is gong on in the center."

The experiment also has yielded precise measures of the some 239,000 miles between the center of the moon and the center of the Earth. The level of accuracy is equivalent to determining the distance between Los Angeles and New York to one-hundredth of an inch.

Shelus explains the widening gap between the Earth and moon as a function of the tides and their friction which slowly, slowly reduce the pace of the Earth's rotation on its axis.

To balance the energy equation, the moon gains spin momentum that takes it 3 to 4 centimeters away from Earth each year, Shelus says.

Shelus, who manages the data collection effort at McDonald Observatory in Ft. Davis, Texas, said the value of the data set is its longevity, allowing scientists to look at finer and finer structures of lunar motion.

He worries that NASA eventually will ax the lunar ranging experiment, which costs about $500,000 a year to run, since its novelty wore off long ago and some might think it has become a bit tired. Shelus disagrees.

"The science is new. The science is exciting," he says. "Over the past 30 years, we have not just rested on our laurels. When you look at the volume and accumulation of data 30 years ago, now you find that the experiment itself has steadily improved over the 30 years."